Vacuum packaging is a well known process for the packaging of a wide variety of food products which involves placing an article inside a thermoplastic film package, removing air from the interior of the package and sealing the thermoplastic film so that the packaging material remains in close contact with the article surfaces when the package is closed. Among the vacuum packaging processes vacuum skin packaging is commonly employed for the packaging of products such as fresh and frozen meat and fish, cheese, processed meat, ready meals and the like. Vacuum skin packaging is described in many references, including FR 1,258,357, FR 1,286,018, AU 3,491,504, U.S. RE 30,009, U.S. Pat. No. 3,574,642, U.S. Pat. No. 3,681,092, U.S. Pat. No. 3,713,849, U.S. Pat. No. 4,055,672, and U.S. Pat. No. 5,346,735.
Vacuum skin packaging is in one sense a type of thermoforming process in which an article to be packaged serves as the mould for a forming web. An article may be placed on a rigid or semi-rigid support generally tray-shaped, bowl-shaped or cup-shaped, having an opening and a rim formed on the periphery of said opening. Said tray-like configuration is generally obtained by a thermoforming step, either in-line or off-line. The supported article is then passed to a vacuum chamber where a length of a film, typically pre-heated in a separate pre-heating station, is positioned above the article placed on the support. Then the film is drawn upward against a heated dome, so that it is fully heated while held by suction in contact with the heated ceiling and walls of the dome. Then, vacuum is applied to the chamber below the film and all around the supported article. As soon as the pressure in the chamber has reached a suitable level below atmospheric pressure, the suction applied to the dome is released and the softened film is pulled downwards to drape over the contours of the article and in contact with the support. The movement of the film is controlled by vacuum and/or air pressure, and in a vacuum skin packaging arrangement, the interior of the container is vacuumized before final welding of the film to the support. The heated film thus forms a tight skin all around the product and it is welded to the support by differential air pressure, thus forming a seal wherever the two surfaces contact each other.
In known vacuum skin packaging processes the film is supplied to the vacuum chamber in the form of a continuous web of film drawn from a roll. The film is cut to the size of the support either within the vacuum chamber during the vacuum packaging process, or at the end of it once the package leaves the vacuum chamber. In either case an excess of the film with respect to the size of the support is fed to the vacuum chamber. The film excess is required to allow the film to be pulled from the roll and to be held in place above the supported article so that air can be removed from within the support. In general, more than one product loaded support is fed to the vacuum chamber at each cycle, typically 2, 3, 4 or even 6 supports at a time, so that an excess of film is also present between adjacent supports. The film is held above the product loaded supports by means of gripping chains, clamps, frames or equivalent holding means. At the end of the packaging process the excess film, which can be as much as 30% or 40% of the total amount of film on the roll, is cut from the package and scrapped.
The need to hold the film in place above the supported article is due to the fact that removal of air from the interior of the support is possible only as long as the film is held above the support and the product and in contact with the dome by suction. As soon as the film contacts the surface of the support along a closed line air can no longer be removed from within the support. Thus, particularly when a deep tray is used as a support for the product, pockets of air may remain entrapped between the film and the bottom surface of the support. The air pockets may negatively influence the shelf-life of the product as well as the impression that the consumer has of the package.
Skin packaging processes wherein the support is provided with perforations or vents to evacuate the air inside the support have been disclosed. U.S. Pat. No. 3,481,101 discloses a method for making skin packages using a support of an impervious material provided with apertures. According to this method once the product to be packaged is placed over the support held above an evacuation platen, a sheet of heated thermoplastic film is draped over the product and the support and vacuum is applied to the underside of the support to hermetically seal the film to the support. The package obtained with this method is not under complete vacuum. In particular, when the support is in the form of a tray, with a bottom wall and an upwardly extending side walls the film is drawn only partially into the tray, and does not form a skin on the product. EP-A-320,294 similarly discloses a skin packaging method wherein a product loaded tray provided with a vent in its side wall is placed on a vacuum platen; an excess of thermoplastic film is held over the tray by a frame and heated until it starts to sag over the product; then vacuum is applied from below the tray to pull the film to conform to the surface of the product and over and around the rim of the tray; the excess film is then trimmed.
Thus the need still exists for a vacuum skin packaging process that does not generate any residual waste material. The need also exists for a vacuum skin packaging process that allows the removal of air from within the support even after the film has contacted the support, to reduce the risk of leaving residual air pockets in the package.
A vacuum skin packaging process includes the following steps. A vacuum chamber is provided having an upper portion opposing a lower tray-holding portion. A semi-rigid or rigid tray loaded with a product is placed in the lower tray-holding portion of the vacuum chamber. The tray has a bottom wall and a circumferential side wall upwardly extending from the bottom wall and terminating in an outwardly projecting rim. The side wall has at least one hole. The film is heated while in contact with the upper portion of the vacuum chamber. Air is evacuated from within the tray through the at least one hole while the film is in contact with the upper portion of the vacuum chamber. Air is introduced to push the film from contact with the upper portion to contact the product and weld the film around the product to the whole of the inner surface of the tray not occupied by the product and to close the at least one hole in the side wall.
Accordingly, a first objective of the present invention is to provide a vacuum skin packaging process in which the removal of air from within the support can continue even after the film has come into contact with the surface of the support. A second objective of the present invention is to provide a vacuum skin packaging process that does not require the use of an excess of the film to produce a package.